The repair and/or cleaning of surfaces is time-consuming and costly, and there is therefore great economic interest in giving surfaces water and/or dirt repellency properties. Adhesion and/or wetting mechanisms are generally governed by surface-energy parameters between the two contacting surfaces. As a general rule, a system attempts to lower it's free surface energy, generally by means of polar or chemical interactions. Where the free surface energies between two components are already inherently very low, it is generally the case that the adhesion between these two components will be weak, as the sum of the low free surface energies is low. As such, in these instances, it is generally beneficial to lower the free surface energy. In the case of pairings with a high and a low surface energy, a factor is the possibilities for interaction. For example, when water is applied to hydrophobic surfaces, it is not possible to induce any marked lowering of the surface energy. This is evident from the fact that the wetting is poor. Non-stick materials, such as perfluorinated hydrocarbons (e.g. TEFLON®), have a very low surface energy but at the same time have virtually no possibilities of interaction with most other substances, whether they are polar or non-polar. Accordingly, few components adhere to such surfaces, and components that have been deposited on these surfaces may be removed very easily.
Nevertheless, these non-stick materials are not capable of suppressing van der Waals interactions, which generally are always active. Accordingly, the adhesion of a component to such a surface is defined substantially by the contact area: in other words, the smaller the contact area, the less the adhesion.
Nature makes use of this phenomenon in order to achieve very low levels of adhesion with respect to water. Thus cabbage leaves and some types of fruit are covered by small wax bumps which, in the non-wetting case, reduce the van der Waals contact area of a water droplet and thus generally the adhesion of the droplets to the leaves, such that the water droplets do not adhere well. When particles of dirt or dust are enclosed in these water droplets, the particles are carried by the droplets off of the surface. This effect is known as the “Lotus effect” (which is so named due to the wax excretions that occur on lotus leaves). Several prior art references utilize the Lotus-effect for providing self-cleaning surfaces. In some embodiments, these surfaces have a hydrophobic microcrystalline structure with protrusions such that water will not adhere to the surface. When such a surface is contacted by water droplets, the droplets absorb dust particles, which may have settled on the surface, and carry them away.
Oils or surfactants, however, will adhere to such hydrophobic surfaces since the enlargement of the surface area increases the effective van der Waals interface. For this reason, the Lotus-effect cannot be used to produce items that repel fats and oils.